Bone growth device and method

ABSTRACT

An intramedullary lengthening device includes a housing and a distraction shaft. The intramedullary lengthening device is placed within a cavity of two bone sections (either already separated or purposely separated for insertion of the device). The distraction shaft of the intramedullary lengthening device is attached to the one of the bone sections using, for example, one or more attachment screws. The housing of the intramedullary lengthening device is attached to the second bone section using, for instance, one or more attachment screws. Over the treatment period, the bone is continually distracted, creating a new separation into which osteogenesis can occur. In one embodiment, the intramedullary lengthening device includes an actuator and an extension rod, which can be attached to one other.

RELATED APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57.

FIELD OF THE INVENTION

The field of the invention generally relates to medical devices fortreating conditions involving the skeletal system and in particular bonegrowth applications.

BACKGROUND OF THE INVENTION

Distraction osteogenesis, also known as distraction callotasis andosteodistraction has been used successfully to lengthen long bones ofthe body. Typically, the bone, if not already fractured, is purposelyfractured by means of a corticotomy, and the two segments of bone aregradually distracted apart, which allows new bone to form in the gap. Ifthe distraction rate is too high, there is a risk of nonunion, if therate is too low, there is a risk that the two segments will completelyfuse to each other before the distraction period is complete. When thedesired length of the bone is achieved using this process, the bone isallowed to consolidate. Distraction osteogenesis applications are mainlyfocused on the growth of the femur or tibia, but may also include thehumerus, the jaw bone (micrognathia), or other bones. The reasons forlengthening or growing bones are multifold, the applications including,but not limited to: post osteosarcoma bone cancer, cosmetic lengthening(both legs-femur and/or tibia) in short stature ordwarfism/achondroplasia; lengthening of one limb to match the other(congenital, post-trauma, post-skeletal disorder, prosthetic kneejoint), non-unions.

Distraction osteogenesis using external fixators has been done for manyyears, but the external fixator can be unwieldy for the patient. It canalso be painful, and the patient is subject to the risk of pin trackinfections, joint stiffness, loss of appetite, depression, cartilagedamage and other side effects. Having the external fixator in place alsodelays the beginning of rehabilitation.

In response to the shortcomings of external fixator distraction,intramedullary distraction nails have been surgically implanted whichare contained entirely within the bone. Some are automaticallylengthened via repeated rotation of the patient's limb. This cansometimes be painful to the patient, and can often proceed in anuncontrolled fashion. This therefore makes it difficult to follow thestrict daily or weekly lengthening regime that avoids nonunion (if toofast) or early consolidation (if too slow). Lower limb distraction ratesare on the order of one millimeter per day. Other intramedullary nailshave been developed which have an implanted motor and are remotelycontrolled. The motorized intramedullary nails have an antenna whichneeds to be implanted subcutaneously, thus complicating the surgicalprocedure, and making it more invasive. These devices are thereforedesigned to be lengthened in a controlled manner, but due to theircomplexity, may not be manufacturable as an affordable product. Othershave proposed intramedullary distractors containing and implantedmagnet, which allows the distraction to be driven electromagnetically byan external stator (i.e., a large electromagnet). Because of thecomplexity and size of the external stator, this technology has not beenreduced to a simple and cost-effective device that can be taken home, toallow patients to do daily lenthenings.

SUMMARY OF THE INVENTION

In a first embodiment, a lengthening device is configured for placementinside or across bone having first and second separate sections. Thedevice includes a housing configured for attachment to one of the firstand second separate bone sections and a distraction shaft having aninternal cavity along a length thereof and configured for attachment tothe other of the first and second separate bone sections. The deviceincludes a permanent magnet configured for rotation relative to thehousing and having at least two poles, the permanent magnet operativelycoupled to a lead screw, the lead screw interfacing with a threadedportion of the internal cavity of the distraction shaft. A thrustbearing is disposed in the housing and interposed between the lead screwand the permanent magnet, the thrust bearing sandwiched between firstand second abutments in the housing.

In a second embodiment, a lengthening device is configured for placementinside an intramedullary canal of a bone having first and secondseparate sections. The device includes a housing configured forattachment to one of the first and second separate bone sections and adistraction shaft having an internal cavity along a length thereof andconfigured for attachment to the other of the first and second separatebone sections. A permanent magnet is disposed in the housing andconfigured for rotation and having at least two poles. A planetary gearset having a plurality of gears is provided, wherein one of the gears isoperatively coupled to the permanent magnet and configured fortransmitting torque, and wherein another gear of the plurality of gearsterminates in an output shaft operatively coupled to a lead screw, thelead screw interfacing with a threaded portion of the internal cavity ofthe distraction shaft.

In a third embodiment, a lengthening system is configured for placementinside an intramedullary canal of a bone. The system includes anactuator with a housing containing a rotatable permanent magnet andmoveable distraction shaft telescopically mounted relative the housing,the moveable distraction shaft operatively coupled to the rotatablepermanent magnet via a lead screw, wherein a distal end of thedistraction shaft is configured for attachment to a first region of thebone and wherein a proximal end of the actuator has a geometricallyshaped hub of a male type. The system further includes an extension rodhaving at one end thereof a geometrically shaped hub of a female typeconfigured to secure to the geometrically shaped hub of the male typedisposed on the actuator, wherein an opposing end of the extension rodis configured for attachment to a second region of the bone.

In yet another embodiment, a lengthening system is configured forplacement inside an intramedullary canal of a bone. The system includesan actuator with a housing containing a rotatable permanent magnet and amoveable distraction shaft telescopically mounted relative the housing,the moveable distraction shaft operatively coupled to the rotatablepermanent magnet via a lead screw, wherein a distal end of thedistraction shaft is configured for attachment to a first region of thebone and wherein a proximal end of the actuator comprises ageometrically shaped hub of a female type. The system further includesan extension rod having at one end thereof a geometrically shaped hub ofa male type configured to secure to the geometrically shaped hub of thefemale type disposed on the actuator, wherein an opposing end of theextension rod is configured for attachment to a second region of thebone.

In still another aspect of the invention, an external adjustment devicefor adjusting an adjustable implant includes a power supply, a controlmodule, and a handheld device comprising at least one permanent magnet.The handheld device is configured to be placed on a first side of apatient's limb and the at least one permanent magnet is configured toturn a cylindrical magnet located inside an adjustable implant. Thecontrol module is configured to restrict the number of turns of thecylindrical magnet located inside the adjustable implant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates side view of an intramedullary lengthening device inplace within a bone according to one embodiment.

FIG. 2 illustrates a side view of the intramedullary lengthening deviceof FIG. 1.

FIG. 3A illustrates a cross-sectional view of the intramedullarylengthening device of FIGS. 1 and 2 taken along the line 3A-3A of FIG.2.

FIG. 3B illustrates a detailed view of the intramedullary lengtheningdevice of FIG. 3A from the area of circle 3B.

FIG. 3C illustrates a cross-sectional view of the intramedullarylengthening device of FIGS. 1 and 2 taken along the line 3C in FIG. 2.

FIG. 4A illustrates a view of several of the internal components of theintramedullary lengthening device of the prior FIGS.

FIG. 4B illustrates a lip seal configured for use in the intramedullarylengthening device of the prior FIGS.

FIG. 5 illustrates a detailed view of several internal components of thedrive mechanism of the intramedullary lengthening device of the priorfigures.

FIG. 6 illustrates a perspective view of an external adjustment device.

FIG. 7 illustrates an exploded view of the magnetic handpiece of theexternal adjustment device of FIG. 6.

FIG. 8 illustrates a cross-sectional representation of a prior artelectromagnetic external device being positioned around a patient'slower thigh.

FIG. 9 illustrates a cross-sectional representation of the externaladjustment device handpiece of FIGS. 6 and 7 being positioned on apatient's lower thigh.

FIG. 10 illustrates a sterilizable kit for use with a modularintramedullary lengthening device.

FIG. 11 illustrates a modular intramedullary lengthening deviceaccording to one embodiment.

FIG. 12 illustrates one end of the actuator of the intramedullarylengthening device of FIG. 11.

FIG. 13 illustrates an extension rod of the modular intramedullarylengthening device.

FIG. 14 illustrates a second view of the extension rod of FIG. 13.

FIG. 15 illustrates a proximal drill guide for insertion and attachmentof the modular intramedullary lengthening device.

FIG. 16 illustrates a removal tool for removal of the modularintramedullary lengthening device.

FIG. 17 illustrates a torque limiting driver for attaching the extensionrod to the actuator of the modular intramedullary device.

FIG. 18 illustrates a section of the actuator of the modularintramedullary lengthening device.

FIG. 19 illustrates a gap (G) between a magnetic handpiece and anintramedullary lengthening device.

FIG. 20 illustrates a locking screw driver for use with theintrameduallary lengthening device.

FIG. 21A illustrates a locking screw for use with the intramedullarylengthening device.

FIG. 21B illustrates the locking screw of FIG. 21A taken along line21B-21B of FIG. 21A.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIG. 1 illustrates the side view of an intramedullary lengthening device110 which has been placed through a hole or bore 108 contained within abone 100. The hole or bore 108 may be made by drilling, reaming and thelike and may extend through both cortical bone (at the end) and throughcancellous (spongy) bone. The intramedullary lengthening device 110illustrated in FIG. 1 includes a housing 112 and a distraction shaft114. In order to grow or lengthen the bone 100, the bone 100 either hasa pre-existing separation 106 or is purposely cut or broken to createthis separation 106, dividing the bone into a first section 102 and asecond section 104. The cut may be done prior to inserting and securingthe intramedullary lengthening device 110, or may be done after thedevice 110 is inserted, for example by use of a flexible Gigli saw. Thedistraction shaft 114 of the intramedullary lengthening device 110 isattached to the first section 102 using one or more attachment screws118. Fasteners other than screws 118 known to those skilled in the artmay also be used to secure the distraction shaft 114 to the firstsection 102 of the bone 100. The housing 112 of the intramedullarylengthening device 110 is secured to the second section 104 of bone 100using one or more attachment screws 116. Again, fasteners other thanscrews 116 may be used to secure the housing 112 to the second section104 of bone 100.

Over the treatment period, the bone 100 is continually distracted,creating a new separation 106, into which osteogenesis can occur.Continually distracted is meant to indicate that distraction occurs on aregular basis which may be on the order of every day or every few days.An exemplary distraction rate is one millimeter per day although otherdistraction rates may be employed. That is to say, a typical distractionregimen may include a daily increase in the length of the intramedullarylengthening device 110 by about one millimeter. This may be done, forexample, by four lengthening periods per day, each having 0.25 mm oflengthening. The intramedullary lengthening device 110, as will be shownin the following FIGS., has a magnetic drive system, which allows thedistraction shaft 114 to be telescopically extended from the housing112, thus forcing the first section 102 and the second section 104 ofthe bone 100 apart from one another. As the distraction is performed, aportion of the housing 112 is able to slide within the hole or bore 108of the first section 102 if bone 100 within a displacement section 120.The orientation of the intramedullary lengthening device 110 within thebone may be opposite of that shown in FIG. 1. For example, thedistraction shaft 114 may be coupled to the second section 104 of thebone 100 and the housing 112 may be coupled to the first section 102 ofthe bone 100. For example, the intramedullary lengthening device 110 maybe placed retrograde, from a hole or bore starting at the distal end ofthe bone 100.

Turning to FIGS. 2 through 5, the intramedullary lengthening device 110has one or more distraction shaft screw holes 122 in the distractionshaft 114 through which the screws 118 (FIG. 1) may be placed. Likewise,the housing 112 is attached to an end cap 130 which has one or morehousing screw holes 124 through which the screws 116 (FIG. 1) may beplaced. The housing 112 of the intramedullary lengthening device 110includes a magnet housing 128 and a splined housing 126. These housings126, 128 may be attached to each other by means of welding, adhesivebonding or other joining techniques. The magnet housing 128 is sealablyclosed at one end (the end opposite the interface with the splinedhousing 126) by the attachment of the end cap 130. The end cap 130 maybe attached to the magnet housing 128 by means of welding, adhesivebonding or other joining techniques. In use, the distraction shaft 114is driven from the housing 112 by means of a lead screw 136 which turnsinside a nut 140 that is secured to an inner surface adjacent to acavity 137 of the distraction shaft 114. The lead screw 136 ismechanically coupled, in an indirect manner, to cylindrical permanentmagnet 134 contained within the magnet housing 128. As explained in moredetail below, rotation of the cylindrical permanent magnet 134, which ismagnetically driven by an external adjustment device 180 (FIG. 6),effectuates rotation of the lead screw 136.

Cylindrical magnet 134 is fixedly contained within a magnet casing 158using, for example, an adhesive such as an epoxy. The magnet casing 158rotates relative to the magnet housing 128. The cylindrical magnet 134may be a rare earth magnet such as Nd—Fe—B and may be coated withParylene or other protective coatings in addition to being protectedwithin the magnet casing 158, for example hermetically potted withepoxy. The magnet casing 158 contains an axle 160 on one end whichattaches to the interior of a radial bearing 132. The outer diameter ofthe radial bearing 132 is secured to the interior of the end cap 130.This arrangement allows the cylindrical magnet 134 to rotate withminimal torsional resistance. At its other, opposing end, the magnethousing 158 includes an axle 161, which is attached to a first planetarygear set 154. The axle 161 includes the sun gear of the first planetarygear set 154, the sun gear turning the planetary gears of the firstplanetary gear set 154. The first planetary gear set 154 serves toreduce the rotational speed and increase the resultant torque deliveryfrom the cylindrical magnet 134 to the lead screw 136. A secondplanetary gear set 156 is also shown between the first planetary gearset 154 and the lead screw 136, for further speed reduction and torqueaugmentation. The number of planetary gear sets and/or the number ofteeth in the gears may be adjusted, in order to achieve the desiredspeed and torque delivery. For example, a lead screw with eighty (80)threads per inch attached to two planetary gear sets of 4:1 gear ratioeach inside a 9 mm device with magnet location in the distal femur canachieve at least 100 lb. of distraction force at a greater than averagedistance or gap from the external device (FIG. 9 or FIG. 19). Theplanetary gear sets 154, 156 output to a planetary gear output shaft144. The planetary gear output shaft 144 extends through a thrustbearing 138 and is secured (by welding and the like) to a lead screwcoupling cap 146. The lead screw 136 is secured to the lead screwcoupling cap 146 by a locking pin 142, which extends through a hole inthe lead screw 136 and holes in the lead screw coupling cap 146. Alocking pin retainer 148 is a cylinder that surrounds the locking pin142, holding this assembly together. Attaching the lead screw 136 to therest of the magnet/gear assembly in this manner, assures that the designis not over-constrained, and thus that the lead screw 136 does not gallwith the nut 140. In addition, a biocompatible grease, for exampleKRYTOX, may be used on the moving parts (lead screw, nut, bearings,housing, and distraction shaft) in order to minimize frictional losses.The lead screw 136 is able to freely rotate within a cavity 137 of thedistraction shaft 114, and only need engage with the short length of thenut 140, this feature also minimizing frictional losses.

The thrust bearing 138 serves to protect the magnet/gear assembly of thedrive from any significant compressive or tensile stresses. The thrustbearing 138 consists of two separate races with ball bearings betweenthe two races. When there is a compressive force on the device, forexample, when distracting a bone 100, and thus resisting the tensilestrength of the soft tissues, the thrust bearing 138 abuts against amagnet housing abutment or lip 150 located in the magnet housing 128.Additionally, though the device is not typically intended for pullingbones together, there may be some applications where this is desired.For example, in certain compressive nail applications it is the goal tohold two fractured sections of a bone together. Because the bone 100 mayhave fractured in a non-uniform or shattered pattern, it may bedifficult to determine the desired length of the nail until after it isimplanted and fully attached. In these situations, it can be easy tomisjudge the length, and so a gap may exist between the bones. Byplacing a slightly extended intramedullary device 110 and securing it,the device 110 may be retracted magnetically, after it has been securedwithin the bone fragments, so that it applies the desired compressionbetween the two fragments. In these compressive nail applications, therewould be tensile force on the device 110 and the thrust bearing 138would abut against a splined housing abutment or lip 152. In bothsituations, the thrust bearing 138 and a rigid portion of one of thehousing sections take the large stresses, not the magnet/gear assemblyof the drive system. In particular, the thrust bearing 138 is sandwichedbetween the abutment or lip 150 and the abutment or lip 152.

Turning specifically to FIGS. 4A and 5, the housing components have beenremoved to reveal various internal features, including a collar thatallows sliding of the distraction shaft 114 within the housing 112, andwhich also keeps the distraction shaft 114 from being able to rotatewithin the housing 112. This allows full stability of the bone 100.Distraction shaft 114 contains several axial grooves 166. The grooves166 have semi-circular indentation cross-sections which allow severalballs 164 to roll within them. The balls 164 are trapped within a linearball cage 162. The splined housing 126 which fits over the balls 164 andlinear ball cage 162 has axial grooves 163 (FIG. 3C) along its innerdiameter surface that are similar to the axial grooves 166 of thedistraction shaft 114. In this regard, the balls 164 and the ball cage162 are interposed between the distraction shaft 114 and the splinedhousing 126. Therefore, the balls 164 are held in place by the linearball cage 162, and mechanically lock the respective grooves to eachother, thus impeding rotation of the distraction shaft 114 within thehousing 112. However, the balls 164 are able to roll within the linearball cage 162, thus allowing axial displacement of the distraction shaft114 in relation to the splined housing 126 of the housing 112 with verylow friction. A lip seal flange 168 contains a custom cross-section lipseal 169 (shown in FIG. 4B) which allows a sliding seal between thedistraction shaft 114 and the splined housing 126, thus protecting theinner contents of the entire assembly from the body environment. The lipseal 169 includes a base portion 173, which seals against the innerdiameter of the lip seal flange 168 (and thus the splined housing 126which is attached to the lip seal flange 168). The lip seal 169 alsoincludes protrusions 171 which slidingly seal against the axial grooves166 of the distraction shaft 114. Inner surface 175 of the lip seal 169slidingly seals against the overall outer diameter of the distractionshaft 114. It should also be noted that the lip seal 169 may be madefrom silicone, EPDM or other rubber materials, and may be coated withsilicone oil, to aid in lubricity. Also, the balls, grooves and ballcage may be coated with silicone oil or a liquid perfluorinatedpolyether such as KRYTOX to aid in lubricity. FIG. 5 shows a portion ofthe magnet casing 158 removed so that the South pole 170 and North pole172 of the cylindrical magnet 134 may be illustrated.

FIG. 6 illustrates an external adjustment device 180 which is used tonon-invasively distract the intramedullary lengthening device 110 bymeans of a magnetic coupling which transmits torque. The externaladjustment device 180 comprises a magnetic handpiece 178, a control box176 and a power supply 174. The control box 176 includes a control panel182 having one or more controls (buttons, switches or tactile, motion,audio or light sensors) and a display 184. The display 184 may bevisual, auditory, tactile, the like or some combination of theaforementioned features. The external adjustment device 180 may containsoftware which allows programming by the physician. For example, thephysician may desire that the patient take home the external adjustmentdevice 180 in order that the patient or member of the patient's familyor friends make daily distractions of the intramedullary lengtheningdevice 110 implanted in the patient. However, the physician is able tokeep the person operating the external adjustment device 180 from overdistracting the patient by programming this into the control box 176.For example, the physician may pre-program the control 176 box so thatonly one (1) mm of distraction is allowed per day. The physician mayadditionally pre-program the control box 176 so that no more than 0.5 mmmay be distracted during any two hour period, or that no more than 0.25mm may be retracted during a five minute period. Settings such as thesemay serve to assure that the patient not be capable of causing severedamage to the bone or tissue, nor disrupt the lengthening process.

Preferably, such instructions or limits may be pre-programmed by thephysician or even the manufacturer in a secure fashion such that usercannot alter the pre-programmed setting(s). For example, a security codemay be used to pre-program and change the daily distraction limit (orother parameters). In this example, the person operating the externaladjustment device 180 will not be able to distract more than one (1) mmin a day (or more than two mm in a day), and will not have the securitycode to be able to change this function of the external adjustmentdevice 180. This serves as a useful lockout feature to preventaccidental over-extension of the intramedullary lengthening device 110.The safety feature may monitor, for example, rotational movement ofmagnets 186 of the external adjustment device 180, described in moredetail below, or the safety feature may monitor rotation of thecylindrical magnet 134 in the intramedullary lengthening device 110, vianon-invasive sensing means.

FIG. 7 shows the detail of the magnetic handpiece 178 of the externaladjustment device 180, in order to elucidate the manner that the magnets186 of the external device serve to cause the cylindrical magnet 134 ofthe intramedullary lengthening device 110 to turn. As seen in FIG. 7,there are two (2) magnets 186 that have a cylindrical shape. The magnets186 are made from rare earth magnets. The magnets 186 may have the sameradial two pole configuration as the cylindrical magnet 134 seen in FIG.5. The magnets 186 are bonded or otherwise secured within magnetic cups187. The magnetic cups 187 include a shaft 198 which is attached to afirst magnet gear 212 and a second magnet gear 214, respectively. Theorientation of the poles of each the two magnets 186 are maintained inrelation to each other by means of the gearing system (by use of centergear 210, which meshes with both first magnet gear 212 and second magnetgear 214). For example, it may be desired that the south pole of one ofthe magnets 186 is facing up whenever the south pole of the other magnet186 is facing down. This arrangement, for example, maximizes the torquethat can be placed on the cylindrical magnet 134 of the intramedullarylengthening device 110.

The components of the magnetic handpiece 178 are held together between amagnet plate 190 and a front plate 192. Most of the components areprotected by a cover 216. The magnets 186 rotate within a static magnetcover 188, so that the magnetic handpiece 178 may be rested directly onthe patient, while not imparting any motion to the external surfaces ofthe patient. Prior to distracting the intramedullary lengthening device110, the operator places the magnetic handpiece 178 over the patientnear the location of the cylindrical magnet 134 as seen in FIG. 9. Amagnet standoff 194 that is interposed between the two magnets 186contains a viewing window 196, to aid in the placement. For instance, amark made on the patient's skin at the appropriate location with anindelible marker may be viewed through the viewing window 196. Toperform a distraction, the operator holds the magnetic handpiece 178 byits handles 200 and depresses a distract switch 228, causing motor 202to drive in a first direction. The motor 202 has a gear box 206 whichcauses the rotational speed of an output gear 204 to be different fromthe rotational speed of the motor 202 (for example, a slower speed). Theoutput gear 204 then turns a reduction gear 208 which meshes with centergear 210, causing it to turn at a different rotational speed than thereduction gear 208. The center gear 210 meshes with both the firstmagnet gear 212 and the second magnet gear 214 turning them at a ratewhich is identical to each other. Depending on the portion of the bodywhere the magnets 186 of the external adjustment device 180 are located,it is desired that this rate be controlled, to minimize the resultinginduced current density imparted by magnet 186 and cylindrical magnet134 though the tissues and fluids of the body. For example a magnetrotational speed of 60 RPM or less is contemplated although other speedsmay be used such as 35 RPM or less. At any time, the distraction may belessened by depressing the retract switch 230. For example, if thepatient feels significant pain, or numbness in the area beinglengthened.

A cross section of a patient's lower thigh 218 with the intramedullarylengthening device 110 implanted within the femur 220 is shown in FIGS.8 and 9. In FIG. 9, the magnetic handpiece 178 of the externaladjustment device 180 of the invention is shown in position to adjustthe cylindrical magnet 134 of the intramedullary lengthening device 110.In FIG. 8, however, a scale depiction of a prior art magnetic stator“donut” 222 demonstrates the comparative efficiency of the two designs(FIG. 8 illustrates an intramedullary lengthening device 110 of the typedescribed herein placed in a “prior art” magnetic stator “donut” 222).The prior art magnetic stator “donut” 222 is large, expensive, anddifficult to transport to a patient's home for daily adjustments. Inaddition, the use of a circular cross-section as a one-size-fits-alldevice is not very efficient because of several reasons: the crosssection of most limbs is not circular, the bone is usually not centeredwithin the limb and patients' limbs come in many different sizes. InFIG. 8, the thigh has been placed through the circular hole in themagnetic stator “donut” and the posterior portion 232 of the thigh restsat the lower portion 226 of the magnetic stator “donut” 222. Thestrength of a magnetic field decreases in accordance with a power (suchas the inverse square) of the distance, depending on the complexity ofthe specific field geometry. Therefore, in any magnetic design, makingthe distance between the driving magnetic field and the driven magnet assmall as possible is desirable. The size of the patient's lower thigh218 and the decision to how it is placed within the magnetic stator“donut” 222 in FIG. 8 create a geometry so that the distance L₁ betweenthe cylindrical magnet 134 and the upper portion 224 of the magneticstator “donut” 222 is about the same as the distance L₂ between thecylindrical magnet 134 and the lower portion 226 of the magnetic stator“donut” 222. However, if the anterior portion 234 of the thigh wereinstead placed against the upper portion 224 of the magnetic stator“donut” 222, the length L₁ would become less while the length L₂ wouldbecome greater. Because each patient has a different sized limb, andbecause small limbs like the upper arm as well as large limbs such asthe upper leg are desired for treatment, the magnetic stator “donut” 222of FIG. 8 is almost impossible to optimize. Therefore, an extra largemagnetic field needs to be generated as the standard magnetic field ofthe device, thus requiring more expense (for the hardware to power thislarger field). This in turn means that each patient will be exposed to alarger magnetic field and larger tissue and fluid current density thanis really required. It may be desired, in some embodiments, to maintainpatient exposure to magnetic fields of 2.0 Tesla or less duringoperation of the device. It may also be desired, according to anotherembodiment, to maintain patient exposure of the patient's tissues andfluids to current densities of no more than 0.04 Amperes/meters² (rms).In addition, because the intramedullary lengthening device 110 issecured to the bone 100, unnecessarily large magnetic fields may causeunwanted motion of the bone 100, for example in any of the radialdirections of the cylindrical magnet 134. If the magnetic field is toohigh, the patient's leg may be moved out of ideal position, and may evencause the patient some annoyance, including pain.

The configuration of the magnetic handpiece 178 of the externaladjustment device 180 as shown in FIG. 9 optimizes the ability of themagnets 186 to deliver torque to the cylindrical magnet 134 of theintramedullary lengthening device 110, without exposing the patient tolarge magnetic fields. This also allows the cylindrical magnet 134 ofthe intramedullary lengthening device 110 to be designed as small aspossible, lowering the implant profile so that it may fit into thehumerus, or the tibia and femurs of small stature patients, such asthose who might desire cosmetic limb lengthening. As mentioned, a 9 mmdiameter intramedullary lengthening device can deliver 100 lb.distraction force, and even 8 mm and 7 mm devices are contemplated. Thealternating orientation of the two magnets 186 (i.e., north pole of onemagnet 186 corresponding with south pole of the other magnet 186)creates an additive effect of torque delivery to cylindrical magnet 134,and thus maximizes distraction force for any specific cylindrical magnet134 size. Also, the separation (S) between the centers of the twomagnets 186 (for example 70 mm), and the resulting concave contour 238(FIGS. 6 and 7), match with the curvature of the outer surfaces of themajority of limbs, thus making the distances L₃ and L₄ between each ofthe magnets 186 and the cylindrical magnet 134 as small as possible.This is especially aided by the concave contour 238 of the magnetichandpiece 178. Also, skin and fat may be compressed by the magnet covers188 causing an indentation 236 on one or both sides which allows thedistances L₃ and L₄ between each of the magnets 186 and the cylindricalmagnet 134 to be yet smaller.

FIG. 10 illustrates a sterilizable kit 400 containing a plurality ofextension rods 406 which are configured to be attached to an actuator412 (FIG. 11) in order to construct a modular intramedullary lengtheningdevice 410 (FIG. 11). In a one embodiment, the actuator 412 is suppliedsterile, and the extension rods 406 and the remainder of the contents ofthe sterilizable kit 400 are sterilizable by autoclave (e.g., steam),Ethylene Oxide or other methods known to those skilled in the art. Thesterilizable kit 400 contents includes one or more of the extension rods406 and accessories 408 for use in the insertion, attachment, adjustmentand removal of the modular intramedullary lengthening device 410. Thecontents are located within a first sterilizable tray 402 and a secondsterilizable tray 404. Second sterilizable tray 404 and firststerilizable tray 402 have a plurality of holes 405 to allow gas toenter. Other items in the kit 400 will be described in several of thefollowing figures.

Turning to FIG. 11 the assembly of the modular intramedullarylengthening device 410 is shown. The actuator 412 is designed to beplaced in the bone of the patient in the opposite orientation than thatof the intramedullary lengthening device 110 of FIG. 1. Therefore, thedistraction shaft 413 is orientated towards the distal end of the bone(distal is the down direction of FIG. 11). Distal screw holes 415 in thedistraction shaft 413 allow the placement of distal locking screws 420.The distal locking screws 420 (FIGS. 21A and 21B) have proximal threads417 for engaging the bone, while the remainder of the shaft 419 of thedistal locking screws 420 is of a constant diameter for maximum strengthand stability. At the proximal end 421 of the actuator 412 there is ahexagonally-shaped male hub 414 containing a transverse set screw 416,within a threaded hole 429 of the hexagonal male hub 414 (FIG. 12). Theextension rod 406 (FIGS. 13 and 14) has a corresponding hexagonal hole428 or female end into which the hexagonal male hub 414 of the actuator412 is placed. The transverse set screw 416 is nested within thethreaded hole 429 of the hexagonal male hub 414 so that it does notinterfere with the hexagonal hole 428 of the extension rod 406, whenthey are placed together. There are two set screw holes 422 in the wallof the extension rod 406 which are in line with each other. The actuator412 and extension rod 406 are placed together so that the set screwholes 422 extend coaxially with the set screw 416. This allows a malehex 490 of a set screw tightening driver, such as the torque limitingdriver 488 of FIGS. 10 and 17, to be inserted into a hex hole of the setscrew 416. When the torque limiting driver 488 is tightened and ratchetsat its set control torque, the other end of the set screw 416, which iseither threaded or a non threaded peg, inserts into the opposite setscrew hole 422, thus tightly securing the actuator 412 to the extensionrod 406. The set screw holes 422 are sized to allow the male hex 490 tosmoothly clear, but the non-threaded peg of the set screw 416 clear veryslightly, making a static connection that cannot be easily loosenedduring implantation. If desired, bone cement may be placed in annulus ofset screw hole 422, to even further bond set screw 416. Also, a secondscrew may be screwed in behind the head of the set screw into the femalethread that the set screw 416 was originally nested in. The head of thissecond screw will add additional resistance to shear failure of the setscrew 416. In addition, the second screw can be tightened so that itjams into the set screw 416, thus making back-out of the set screw 416unlikely. Any non-circular cross-section may be used in place of the hexcross-section, for example a square or oval cross-section.

Proximal locking screws 418 insert through locking screw holes 430 inthe extension rod 406. The extension rod 406 may be straight, or mayhave a specific curve 432, for example, for matching the proximal end ofthe femur or tibia. It can be appreciated that the modular arrangementallows the actuator 412 to be attached to one of numerous differentmodels of extension rods 406, having different lengths, curves(including straight), diameters, hole diameters, and angulations. Thefirst sterilization tray 402 may include many of these differentextension rods 406, which may be selected as appropriate, and attachedto the actuator 412. Because the actuator 412 is supplied sterile, thisarrangement is also desirable, as only a single model need be supplied.However, if desired, several models of actuator may exist, for example,different diameters (10.5 mm, 12.0 mm, 9 mm, 7.5 mm) or with differentdistal screw hole diameters, configurations or angulations. Thepreferred configuration for a multitude of patients and different bonetypes and sizes can be available, with a minimum number of sterileactuator models.

Turning to FIG. 15, a proximal drill guide 434 is illustrated and isconfigured for attaching to the modular intramedullary lengtheningdevice 410 to ease its insertion into the intramedullary canal, thedrilling of holes in the bone and the attachment of the proximal lockingscrews 418 to the bone. The proximal drill guide 434 comprises anextension arm 436 attached to a connection tube 446 through which alocking rod 448 is inserted. The locking rod 448 has a locking knob 450at the proximal end and a male thread 452 at the distal end. In order totemporarily attach the proximal drill guide 434 to the modularintramedullary lengthening device 410, a locking tab 454 of the proximaldrill guide 434 is inserted into a locking groove 424 of the extensionrod 406 and the locking knob 450 is turned, threading the male thread452 of the locking rod 448 into a female thread 426 of the extension rod406. Prior to the procedure a drill guide extension 438 is attached viaa knob 440 to the extension arm 436. After reaming the medullary canalof the bone to a diameter slightly larger than the outer diameter of themodular intramedullary lengthening device 410 (for example 11 mm),distal end of the modular intramedually lengthening device 410 isinserted into the medullary canal and the flat proximal surface of thelocking knob 450 is hammered with a mallet, allowing the modularintramedullary lengthening device 410 to be inserted to the correctdepth. Dimension X is sufficient to clear large thighs or hips (in theworst case femoral application). For example, 8 to 10 cm is appropriate.Once the modular intramedullary lengthening device 410 is in place inthe medullary canal, the proximal drill guide 434 is left attached and aguide sleeve 442 is placed through one of the holes 456, 458, 460, 462and slid so that the distal end 443 reaches the skin of the patient. Thedrill guide extension 438, extension arm 436 and holes 456, 458, 460,462 are dimensioned and oriented so that the guide sleeve 442 isoriented at the exact angle to allow drilling and placement of screwsthrough the locking screws holes 430 of the extension rod 406 andthrough the bone. The skin of the patient is cut and a drill bushing 444is placed through the incision, with the tapered tip 445 passing throughtissue and reaching the bone to be drilled. For example, drills andlocking screws may be inserted down the drill bushing 444, oralternatively, drills may be inserted down the drill bushing 444 andthen, after the drilling is complete, the drill bushing 444 is removedand proximal locking screw 418 is inserted down the guide sleeve 442.Alternative guide sleeves 464 and drill bushings 466 can be placedthrough holes 460 and 462, as seen in FIG. 10.

Turning to FIG. 16, a removal tool 468 is illustrated. The removal tool468 is used after the distraction period and consolidation period arecomplete. To remove the modular intramedullary lengthening device 410from the medullary canal, the skin is incised and bone exposed at thelocations of the proximal and distal locking screws 418, 420 and at theproximal end of the modular intramedullary lengthening device 410. Aremoval rod 470 is connected to the female thread 426 of the extensionrod 406 of the modular intramedullary lengthening device 410 byinserting the engagement tip 476 and screwing the male thread 474 intothe female thread 426, holding onto the locking knob 472. The lockingknob 472 contains a female thread 478 which allows the attachment of amale thread 486 of a removal extension 480, which has an impact knob 482and removal hammer 484. The male thread 486 is coupled to the removalextension 480 by a pivot 477 of a pivoting base 479. The male thread 486is secured to the female thread 478 by grasping and turning the impactknob 482. Prior to removing the modular intramedullary lengtheningdevice 410, the proximal and distal locking screws 418, 420 are removed.They may be removed with the use of the locking screw driver 498 (FIGS.10 and 20), which has a male hex tip 497 to engage the proximal ends ofthe locking screws 418, 420. A screw capture rod 500 (FIGS. 10 and 20)inserts down the center of the locking screw driver 498 and has a malethreaded tip 501. At a deeper portion past the female hex 513 in thelocking screws 418, 420 (FIGS. 21A and 21B) is a female thread 511. Themale threaded tip 501 of the screw capture rod 500 threads into thefemale thread 511 of the locking screws 418, 420, and tightened by usingthe tightening handle 503 of the screw capture rod 500 which sits at thehandle end 509 of the locking screw driver 498 so that once the lockingscrews 418, 420 are removed from the bone, they are still secured to thelocking screw driver 498, and will not become prematurely displaced. Forexample, the locking screws 418, 420 will not be lost or dropped intothe patient. The modular intramedullary lengthening device 410 may nowbe removed from the medullary canal by grasping the removal hammer 484,and moving it quickly in the direction (D) so that hammer impact surface485 strikes knob impact surface 483. This is done until the modularintramedullary lengthening device 410 is completely removed. It shouldbe noted that locking knob 450 of the proximal drill guide 434 of FIG.15 also has a female thread (not pictured) so that during the insertionof the modular intramedullary lengthening device 410, if it is desiredto remove the device for any reason, the male thread 486 of the removaltool 468 may be attached to the female thread of the locking knob 450,and the removal hammer 484 can be used against the impact knob 482 toremove the modular intramedullary lengthening device 410.

The torque limiting driver 488 of FIG. 17 comprises a handle 496 and ashaft 492 having a torque-specific ratchet 494 connecting them. The malehex tip 490, fits into the hex hole of the set screw 416, or even intothe female hex 513 of the locking screws 418, 420. An exemplaryratcheting torque for the set screw 416 is 9 inch-pounds (1.0Newton-meter), and an exemplary hex size is 1/16″ (1.59 mm).

FIG. 18 illustrates the actuator 412 of FIG. 11 in a sectional view. Thedistal screw holes 415 are visible in the distraction shaft 413. Thedistraction shaft 413 is shown in a fully extended position in relationto the housing 312. The cavity 337 has opened to its maximum length. Inthis embodiment, the distraction shaft 413 has a purely cylindricalsurface, and is dynamically sealed to the housing 312 by two o-ringseals 502. The o-ring seals 502 may be made of silicone, EPDM, or otherrubber materials, and may be coated with silicone oil, to aid inlubricity. There are four axially extending grooves 326 on the innerwall of the housing 312. Tabs 504 on the end of the distraction shaft413 fit into these grooves 326 to keep the distraction shaft 413 frombeing able to rotate with respect to the housing 312. The housing 312 iswelded to a magnet housing 328 and the magnet housing 328 is welded tohexagonal male hub 414. The set screw 416 on the hexagonal male hub 414is used to attach the actuator 412 to the extension rod 406. Thecylindrical permanent magnet 334 is cased with epoxy inside magnetcasing 358 having an end pin 360. The end pin 360 inserts through radialbearing 332, allowing it to rotate with low friction. As the magnet 334is rotated by the external magnets, first planetary gear set 354, secondplanetary gear set 356 and third planetary gear set 357 allow a totalreduction of 64:1 (4×4×4). Each gear set allows a 4:1 reduction.Planetary gear output shaft 344 is attached to lead screw 336 by lockingpin 342, and locking pin 342 is held in place by cylindrical locking pinretainer 348. Thrust bearing 338 abuts housing abutment or lip 352 andmagnet housing abutment or lip 350 (thrust bearing 338 is sandwichedbetween housing abutment or lip 352 and magnet housing abutment or lip350). Therefore, thrust bearing 338 abuts housing abutment or lip 352 intension and magnet housing abutment or lip 350 in compression. It shouldbe noted that the sandwich arrangement allows for some slop or playbetween the thrust bearing 338 and the housing abutment or lip 352 andthe magnet housing abutment or lip 350. Lead screw 336 engages with nut340, which is secured within distraction shaft 413. With the 64:1 gearreduction of this embodiment, distraction forces of greater than 300pounds (1334 Newtons) have been consistently achieved with a gap (G inFIG. 19) of 2 inches (5.08 cm) between the magnetic hand piece 178 andthe intramedullary lengthening device 110. This is sufficient fordistracting a large range of typical patients.

It should be noted that although the embodiments of the intramedullarylengthening devices presented are shown to be used in a preferredorientation (distal vs. proximal), any of these embodiments may be usedwith the distraction shaft pointing distally or proximally. In addition,the invention may also be applied to distractable bone plates that arenot located within the intramedullary canal, but are external to thebone.

An alternative lengthening scheme than those presented above may be alsoused. For example, one alternative includes the purposefulover-lengthening (to further stimulate growth) followed by someretraction (to minimize pain). For instance, each of four daily 0.25 mmlengthening periods may consist of 0.35 mm of lengthening, followed by0.10 mm of retraction.

The materials of the accessories 408 are medical grade stainless steel,though other materials of varying densities may be used depending on thedesired weight and the required size. The majority of the components ofthe intramedullary lengthening devices are preferably Titanium orTitanium alloys although some of the internal components may be madefrom stainless steel.

While embodiments of the present invention have been shown anddescribed, various modifications may be made without departing from thescope of the present invention. As one example, the devices describedherein may be used to lengthen or reform a number of other bones such asthe mandible or the cranium. The invention, therefore, should not belimited, except to the following claims, and their equivalents.

The invention claimed is:
 1. A lengthening device comprising: a housingconfigured to be attached relative to a first bone section, the housinghaving a first abutment and a second abutment about an inner surface ofthe housing; a distraction shaft having a cavity along a length thereofand being configured to be attached relative to a second bone sectionand being telescopically displaceable relative to the housing; and anactuator configured to increase at least one of a distraction forcebetween the distraction shaft and the housing and a length of thelengthening device, the actuator including: a permanent magnet beingconfigured for rotation relative to the housing; a lead screwoperatively coupled to the permanent magnet via a gear assembly; and athrust bearing disposed about a portion of the gear assembly, the thrustbearing being positioned between the first abutment and the secondabutment of the inner surface of the housing.
 2. The lengthening deviceof claim 1, wherein rotation of the permanent magnet causes the actuatorto increase at least one of a distraction force between the distractionshaft and the housing and a length of the lengthening device.
 3. Thelengthening device of claim 1, wherein the gear assembly includes atleast one gear operatively connected between the permanent magnet andthe lead screw.
 4. The lengthening device of claim 1, wherein the gearassembly includes a plurality of intervening gears disposed between thepermanent magnet and the lead screw.
 5. The lengthening device of claim1, wherein the thrust bearing abuts at least one of the first abutmentand the second abutment in the housing.
 6. The lengthening device ofclaim 1, wherein the thrust bearing is configured to protect at leastpart of the actuator from compressive or tensile stresses.
 7. Thelengthening device of claim 3, wherein the thrust bearing is configuredto protect at least one of the permanent magnet and the at least onegear from compressive or tensile stresses.
 8. The lengthening device ofclaim 1, wherein the lead screw interfaces with a threaded portion ofthe cavity.
 9. A lengthening device configured for placement inside anintramedullary canal of a bone having first and second separate sectionscomprising: a housing configured for attachment to one of the first andsecond separate bone sections, the housing having a first abutment and asecond abutment about an inner surface of the housing; a distractionshaft having an internal cavity along a length thereof and configuredfor attachment to the other of the first and second separate bonesections; a permanent magnet disposed in the housing and configured forrotation; an output shaft operatively coupled to a lead screw, the leadscrew interfacing with a threaded portion of the internal cavity of thedistraction shaft; a gear assembly operatively connecting the permanentmagnet and the output shaft; and a thrust bearing disposed about aportion of the gear assembly and thrust bearing being positioned betweenthe first abutment and the second abutment of the inner surface of thehousing.
 10. The lengthening device of claim 9, wherein the gearassembly comprises at least one planetary gear set comprising aplurality of planetary gears.
 11. The lengthening device of claim 10,wherein the gear assembly comprises a first planetary gear setcomprising a plurality of planetary gears and a last planetary gear setcomprising a plurality of planetary gears, wherein rotation of thepermanent magnet turns a sun gear of the first planetary gear set and anoutput of the last planetary gear set is operatively connected to theoutput shaft.
 12. The lengthening device of claim 9, wherein the thrustbearing abuts at least one of the first abutment and the second abutmentin the housing.
 13. The lengthening device of claim 9, wherein thethrust bearing is configured to protect at least part of the actuatorfrom compressive or tensile stresses.
 14. A lengthening devicecomprising: a housing configured to be attached relative to a firstlocation on a bone, the housing having a first abutment and a secondabutment about an inner surface of the housing; a distraction shaftmovable relative to the housing and configured to be attached relativeto a second location on the bone; a drive system comprising: a permanentmagnet configured for rotation relative to the housing; and an outputshaft operatively coupled to the permanent magnet via a gear assembly,wherein rotation of the permanent magnet causes movement of the outputshaft; and a thrust bearing disposed in the housing about a portion ofthe gear assembly between the first abutment and the second abutment ofthe inner surface of the housing, the thrust bearing configured toprotect at least part of the drive system from compressive or tensilestresses.
 15. The lengthening device of claim 14, wherein the drivesystem further comprises at least one gear operatively coupled to thepermanent magnet and the output shaft.
 16. The lengthening device ofclaim 14, wherein the at least one gear comprises at least one planetarygear set comprising a plurality of planetary gears, wherein thepermanent magnet is operatively connected to a sun gear and the at leastone planetary gear set is operatively connected to the output shaft.